Method and apparatus for radiological measurements



July 19, 1949.

E. M. BRUNNER ET AL Filed April 1o, 1945 E. 5. Mar-CCK Patented `uly 19, 1949 .MTHOE AND APPARATUS FOR RADIO- LOGICAL MEASUREMENTS Eugene M. Brunner, El Cerrito, and EdwinS. Mardock, Berkeley, Calif., assignors to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application April 10, 1945Serial No. 587,598

6 Claims.

lThis invention pertains to a method and apparatus for the investigation and determination of the nature, composition and other characteristics of Various bodies or substances by radiological means involving the use of neutrons.

Radiological methods have been applied to various purposes, such as geophysical exploration, for example, in logging oil and gas wells, or in investigating the condition of various industrial elements or structures, for instance, in measuring the wall thickness of metallic sheets, pipes, etc.

In applying these methods, use may be made of diierent types of radiation, such as X-rays, gamma rays, neutrons, etc., the use of neutrons being of especial advantage because of their extremely high penetrating power.

Neutrons may be produced or generated by the action of a radioactive substance such as radium, radon, polonium, etc., on a substance such as beryllium, lithium, etc. For example, beryllium emits neutrons when impinged upon by alpha particles emanating from radium. A radiumberyllium mixture is therefore a convenient source of neutrons. The neutrons emitted by any such source have an extremely high velocity, corresponding to energy values averaging between 1 106 and 1O 106 electron volts, and are called fast neutrons. Fast neutrons have the property of passing freely through heavy elements, that is, elements of heavy atomic weight, which are not pervious to any considerable depth to radiations of other types, such as alpha, beta and even gamma rays. On the other hand, light elements, that is, elements of low atomic weight, and especially hydrogen, have the property of greatly decreasing the velocity of the neutrons. Neutrons whose velocity has been reduced by co1- lision with low atomic weight elements are known as slow neutrons. The passage of neutrons through substances comprising high atomic weight elements thus results principally in scattering c-r diffusing these neutrons, while a passage of neutrons through substances comprising low atomic weight elements results in both slowing down and scattering said neutrons.

Since neutrons carry no electric charge, they cannot be directly detected by ordinary means used for the detection of other radioactive radiations, such more specifically as Geiger-Mueller counters and ionization chambers. However, certain elements or compounds thereof, such as cadmium, lithium, boron, boron triuoride, etc., which are substantially insensitive to the elect of fast neutrons, have the property of disinte- (Cl. Z50-83.6)

grating with an accompanying emission of alpha or gamma particles when impinged upon by slow neutrons. By placing such elements in the vicinity of Geiger counter or ionization chamber detectors, for example, by coating the walls of the latter with cadmium, or by lling them with boron triiluoride, these detectors may be applied for the detection of slow neutrons. Since, for a neutron radiation source of a given intensity, the number of slow neutrons lreaching such detector will be substantially a function of the amount of the hydrogen atoms present in the medium traversed by the path of neutrons to the detector, this amount can be accurately measured by observing or recording the indications of a properly calibrated detector of this type. This method can therefore be advantageously used for determining the presence of substances containing free or combined hydrogen, such as water, hydrocarbon liquids or gases, etc., in cases where more direct measurements are for some reason difcult or impossible.

' It has however now been found that measurements or determinations made in the manner described above `are not always reliable or accurate, due to the fact that sources of neutrons, such as a radium-beryllium mixture, actually emit not only neutrons, but also radiations of other types. Thus, while the .beryllium of such a mixture emits, under the influence of radium, a neutron radiation, the radium itself simultaneously emits alpha, beta and gamma radiations. The response of a neutron detector, comprising for example a Geiger-Mueller counter, is therefore partially due in such cases to the eiect of slow neutrons on the disintegrating element, such as Cadmium, placed about the counter, and partially to the direct effect of gamma rays from the same source on the counter itself. Since, even with the use of lead filters or other suitable devices, it is sometimes impossible to separate neutron and gamma radiations from each other and to differentiate between the detector response due to neutrons and that due to gamma-ray leakage, the indications of said detectors lack in such case the accuracy desired with regard to quantitative hydrogen atom measurements.

It is therefore an object of this invention to provide a method'and an apparatus free of the above drawbacks, whereby the hydrogen content, that is, the amount of free or bound hydrogen atoms present-in a particular substance or body of matter, can be determined, thus yielding the desired qualitative and quantitative measurements with regard to said substance or body.

Figure 2 is a diagrammatic view in cross-section of a test plate used in practicing the present invention;

Figure 4 is an illustrative calibrationy curve such as may be used in practicing-:thepresentinvention. Y

The operation of the presentinvention-:is 'based on the following principles. As stated, slow neu-'.1 trons upon striking certain elementsknoclroi.

or cause the emission of gamma rays, as in the case of cadmium, or alpha particlesassin the;

case of boron or lithium. This may be termed the immediate effect of slow neutron bombard-` ment: Some otherv elements, however, such as silverindium, rhodiu manganese; dysposium, etc.,. doA not :lose e any radioactive particle immediately upon beingA subjected to `neutron impact. Instead, these elements upon receiving a neutron in-their atom structure are converted to an isotopicl form having a different atomic weight but thezsame electriccliarge per atom` astheoriginal Vmatter. This isotopic form being unstable, these elements tend to Aundergo,'within a` certain statistical time period, a furtherfchangerto a more stable form. This change or deactivation from Y oneisotopiciorrn` toanother is accompanied by the: emission of'ionizing particles, and more spe,-

cically of beta particles, and .theperiods through which `it takes place varyifor difierent elements. The. use of the elements listed vabove Vis `especially suitable, rst, because they are susceptible of strongactivation by slow'neutrons and, second, becausetheir decay orwdeactivation periods are Vwell adapted for laboratory measurement purposes. Thus, for example, silverhas-afshorter periodwith a half-lifeof 22 seconds, and a longer period with a half-life of 2.3 minutes.` This may be -termed the delayedd effect'` of neutron bombardment. c

The method Y of the presentxinventionf Yutilizes this delayed effect. to measurev the hydrogen atom ccntent of any desiredbody orxsubstance under investigation, and will be described with @regard to one particular example selected forfpurposes of illustration,l it beingclear that '.themethodis applicable to a variety of othenuses; vamongwhich thefollowing may be given likewiseionly for fpurposes oi illustration: measurementgofthe. amount or level of a hydrogen-containing liquid present in a container in pureform or inxmixture'or combination with other liquids notcontaining hydrogen; measurement of the degreeof moisture present in a particular, material or substance; analysis of the chemical composition of a subrstancewith regard to a particular hydrogenecontaining compound; measurement ofthe hydrogen embrittlement in metallic members such as pipes, plates: etc.; measurement of the pressureofa Vhydrocarbon or other hydrogen-'containing gas in a closed` vessel, etcL Figure Y l illustratesthe applicationof. the rpresent method withV regard tothe measurement of the saturation of a'porous corezI, held-in a'rnetallic pressure barrel 3, through whichwater, oilor any other hydrogen-containing: liquidon gas-is circulated, if desired,V under pressure, by; means of pipes A and 1, `and a1 suitable pumping equip-- ment, not shown. A source of neutrons 9, such as a radium-beryllium mixture in a suitable lead container II, is placed in any desired position in close proximity to the core I. A test member or plate I5, made of or comprising an elementV capable of undergoing suitable isotopic changes under they effect of neutron bombardment, such as rhodium, indium, silver, manganese, dysposium, etc., is likewise placed adjacent and as close as possible to the core I. The plate I5 may be `positioned on theopposite side of the Core I, as

shown in the drawing for clarity, or in any other. angular relationship with the core and the source. It is however preferred to position the plate on thesame side as the source, and speccally between the source and the core, as this arrangement has been found to give several important advantages, such as an improved total countand sensitivity, an improved differentiation between unequally saturated portions of the core, etc.

Thetest lplate may be constructedvin different forms ,or shapes. In Figure 1 it is shown con'- structed as the cylindrical'cathode I5 of a Geiger counter or ionization chamber I1, provided with the-usual anode I9, both the anode and the cathode being held at a: desired pressure within an enclosure 22,.made of glassor other suitable material. The anode and cathodeare electrically connected by Ameans of leads 3| and. 33 to the amplifier, power supply and indicating or record-v ing units 25 and 26.

It will thus beseen that in theembodiment of Figure-Lthe plate I'5 combines thefunctions of a test plate andA of the cathode of the detector. At the beginning ofan activation period, the. detector is rendered inoperative by disconnecting the power supply, which is eiected by turning offa switch 35 provided for this purpose inthe unitl 25, so that` duringthe activation period the plate I5serves-only as the test plate.

The radiation from source 9 comprises fast neutrons and gamma rays. The gamma rays .are of4 no consequence forrthe present method, and cannotfurthermore exert any harmful action on the detector, which is inoperative and gives no indications during the activation period.y The fast neutrons penetrate the core holder 3 and thecore I, being subjected therein to the usual scattering eiect by` collision with heavy atoms and` being vat, the same time scattered and converted to slow neutrons by collision with the hydrogen atoms of the liquid saturating the core i. The slow neutrons, scatterngin all directions from-corel, impinge upon. the plate I5 and activatesaid plate by transforming the metal thereof into an isotope. This activation phase or period iscontinued luntilthe platev I5-is activated substantiallyfully orl as fully as desired; Since the activation of an.f element proceeds asymptotically at approximately the same rate as its subsequent deactivation, thatY is, its conversion to a more stable condition. upon the removal of the energizing source, the activation period may be readilyselected onthe basis of half-life period values of the element of the plate I5. To avoid errors ingtimingmound. gures for `the activation period, suchfor example as 5 minutesfor silver or'rhodium, may conveniently befselected.

Whenthe plate I5 has been substantially completelyv activated, .the source 9 is quickly removed to aplace sufficiently distant from the detector to preclude the ypossibility of any' radioactive eiect thereon, and the detector I1 isput in voperatiomby,v closingthe switch 35. Y

As the plate I5, being free from the effect of the source S, undergoes its isotopic change to a moresstable state, which change is accompanied by the'specific emission of ionizing beta particles, these particles produce their usual elect of causing a current or current pulses to pass between the electrodes l and I9 of the detector.

It must be particularly pointed out that the reason for building in the test plate i5 into the `detector as the cathode thereof is that the penetration power. of beta particles, on which the operativeness 'of the present method is based, is extremely small as-compared with that of other types or". radiations, such as neutrons or gamma rays. vThe' present arrangement, wherein the beta particles do not have to pass through any layer or layers ofsolid material, as would be the case if the test plate were arranged exteriorly of the detector, is therefore effective in increasing the sensitivity of the system.

It is however possible, and sometimes even desirable according to the present invention, to use test plates which do not form a part of the detector structure. Thus, the plate I5 may be replaced with a test plate of any desired shape or ioim, such as a at plateY I5A, shown in Figure 2, which may be positioned and subJected to energization in the same manner as already described with regard to plate l5, it being understood that in such cases the detector I1 and the units 25 and 26 are removed from the proximity of the core l and source 9.

When the plate A has been substantially completely activated, it is removed from the proximity of the core l and arranged to act on the detecting and measuring apparatus of Figure 3, which is sumciently remote from the sphere of source to be free of any radioactive effects of the latter.

The plate 15A is placed in close proximity to a detector li, which is likewise of the Geiger- Mueller counter or ionization chamber type. As the plate ISA, being removed from the effect of the source s", undergoes its isotopic change accompanied by the specic emission of ionizing beta particles, these beta particles penetrate the detector itl, causing the latter to give its conventional response in the form of a current or current pulses, known as counts, passing between the anode #is and the cathode 4l thereof.

In view of the low penetration power of the beta particles, the usual fairly thick outer walls or envelope of the conventional detector 47 are provided with a thinned portion or window 43 made of a material easily pervious to beta-particles, such for example as a thin sheet of mica, which may be supported for mechanical strength by an internal or external lattice work diagrammatically shown at 44 and made of metal or other suitable material.

The current or current pulses or discharges originating in detectors l1 or `4l are transmitted to an amplier unit 25, which comprises also a power unit, supplying the necessary operating voltages for both the detector and the ampliiier. The amplified pulses are then transmitted to an indicating unit 26, which indicates, registers or records the number of pulses received throughout a predetermined time interval. Since the number of such pulses per unit time may sometimes be relatively high, tending to overload the apparatus, the indicating unit may be provided with a scaling unit adjustably adapted to indicate only every 4th, 8th, 16th or 32nd pulse.

Since the number of pulses produced by the detector in a predetermined time period is proportional to the degree of activation of the plates l5 or 15A, which is in turn proportional to the amount or" slow neutrons impinging upon said plates during theactivation period, and since the number of these slow neutrons formed by collision with hydrogen atoms is a function of the number of said hydrogen atoms, and therefore of the quantity of the hydrogen-containing liquid saturating the core, this saturationcan be easily determined by. the present-method after .properly Calibrating the apparatus used. u

Thus, as an example, the core l may be saturated with kerosene to a saturation, vand the test plate activatedfor 5 minutes with the apparatus arranged as shown in Figure 1. Fifteen seconds after the end of the' activation period, the counting period is started by switching on and operatively energizing the apparatus of Figs. 1 or 2. If, for example, a count of 685 Apulses is registered for the first minute, av count of 320 for the second minute, and a count of 210 for the third minute, and the normal background noise or count of the apparatus under the conditions used is approximately 200 counts per minute, the counting is discontinued after 3 minutes, and the measurements repeated 5 times, a count of 1225 for a period of 3 minutes being nally obtained as an average total count. The operations are then repeated for decreasing known saturations of the core I, until, for example, an Vaverage total of 620 counts is obtained with the core completely dry. The results of this calibration are plotted in Figure 3. If now it is desired to measure an unknown degree of saturation of the core l with the same liquid, it is only necessary to repeat the procedure described, and to refer the number of counts registered to the above curve. Assuming, for example, that such count is 915, the saturation of the core is found to be 65%. 'raking 0.675 VTI as the probable statistical error in counting N pulses, the probable error in a single run will be 20.25 counts or 2.2%. If the measurements of the unknown saturation are repeated 5 times, and 915 is found to be the average total of the counts registered, the probable error is further reduced by a factor of \/5, giving a probable error or' only 1%.

It is obvious that instead of determining the saturation of a core within the barrel 3, the method described above may be used to determine the pressure of a hydrogen-containing gas or gas mixture supplied thereto by means of pipes 5 and 1, the level of a hydrogen-containing liquid standing therein, etc.

We claim as our invention:

1. In a method for determining the pressure of a gaseous uid comprising a hydrogen atom in its molecular structure, said gaseous uid being confined in a solid container, the steps of subjecting said container to neutron radiation throughout a predetermined time period, whereby neutrons penetrating said container are slowed down by .L`2'.`Inv.a methbdforfdetemiiningithe leilmf a liquid confined in a container,r said. liquid `com .pri-sing hydrogen ;atoms Ain its molecular istmo.- fture,v the steps .of .subjecting Vsaid, container ,tmouglnout.el .predetermined period of time .to :radiationzfrom a radioactive -sourcesimultaneonsly `emitting .fast Hneutrions yand iradiations of .other .ty-pes, wherebywheiast l neutrons Lpenetrat- 'ing said container are slowed .-dcwn by collision with Ihydrogen latoms, present therein yand are scattered by collision withsatpmszof heavieriele- -mentsfalong thepath ofV thezneutrons,..ca1ising a portionof slow; neutrons scattering. from said conitaineri-to strike :a materiall capable Rcf undergoing yan isotopic chan-gedlie to the effectoslownell- :trous .on its latomic k:structurerV thereby .activating said :material to an isotopic iform, ,discontinuing said activationgby separating "saidY source. and Said :material inispac'e lbyea: distance suilcient to prev @vent lthe ,--radiation `vof,` said source from reaching said materia1,.permitting said material toreadjust -its `atomic vstructure during -apredetermined deac- `titration;pericdileyaemitting ionizing beta particles, and registering :the :number of `said vionizing `par-- lticles emitted by said material during said deac- .ti-'vation :periodY f v .3. Thefmethodioftclaim 6; wherein the material capable of undergoingfis'otopic f changes `is silver. 14. The methodofelaim 6, lwherein thematerial capable of undergoing isotopicYfchanges-is indium. f5. Themethodfof claim-6, wherein' the material 4capable :of A-un'clerfgoing isotopic ychanges 'is Adyszposium.

Ingamethod ,for rleterrriiningl .the concentra-` tion. ofihydrogendatoms present ,in a fluid psub- `stance .withina*,connedspace, the. steps oLsub- :j ecting, said substance, to neutron irradiation throughout apredeterminedperiodlof,time,y caus- .instne meutronsslowed by ,passage .through said substance. ,to activate a teammaterial capable of undergoing isotopic 4cl'ianges under .slow neutron vinfile-ct,..discontinuingesaid,neutrcri .radiatiomand registering the :numberpfpionizingbeta; particles emttedaby said material throughouta subsequent predetermined .observation Yperiod.

- Y EIIGENE;M. BRUNNER.

REFERENCES .CITED The 'follofwing -referemces 4are of -record yin'fthe Number 'iNarne Y. Datel 2,296,634 Fermi et al. Y v v July. 2, 1940 12,220,509 v .Brons \NoV.-5,\1940 2,288,718 .Kallmann-et-al July `7, 1942 s 2,303,688 Y Y Fearon Dec. 1,:1942 ,2,304,910 .Hare Dec. 15, 1942 2,373,219 Hare f 1 June 12, 1945 'OTHER 'REFERENCES' Livingood and Seaborg. article Ain Reviews oi Modern Physics, Jan. ,1940, pp.,30 and 34-,43. 

